Design Process of Supersonic Isentropic Inlet with Expansion Wave Concept

Abstract

The supersonic isentropic inlet is a type of inlet that is easy to be adopted to hypersonic and supersonic flying vehicle configuration and suitable to work over a large range of flying states. The inlet has a sound performance in terms of total pressure recovery and flow coefficient owing to its compact design to decrease the negative influences of shock waves. However, it still remains a tough task to design the inlet for the time being. It is here introduced in the paper the concept of expansion waves to simplify the design process of isentropic inlet. It is known to all that the air captured by the inlet will gradually decrease its speed due to influence of the countless numbers of compression waves. However, as we observe in the opposite direction, the air will accelerate its speed gradually in expansion waves flow field. One thing will remain the same in the preceding two flow situations, namely, streamline. Based on the idea, the profile traced in expansion waves flow field can yield the compression surface of isentropic inlet. Expansion waves are phenomena that are very common in supersonic flows. Certain types of expansion waves in a steady uniform parallel two-dimensional flow, which are called Prandtl-Meyer waves, can be analyzed by simple equations. With the joint solution of the MOC equations and Prandtl-Meyer equations, the supersonic expansion waves flow field, serving as the base flow field of designed inlet, can be obtained precisely. Next, the introduction of bilinear interpolation method allows the calculation of each point on a streamline starting from a given initial point. Finally, a streamline-traced sketch, derived from the above base flow field, will be the desired inlet forebody. The following two-dimensional numerical simulation conducted accordingly indicates the actual performance well satisfy the designed state. The fact that the designed inlet has a good performance in terms of total pressure recovery means that the design method is practical and feasible, and broadens applications of design method of such inlets.